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Yes, when engine coolant temp is under about 140degF computer runs Open Loop
But only O2 sensors are not used, O2 sensors need to be at or above 600degF to read oxygen levels correctly, which is why O2s are also heated, to get them up to temp ASAP
Computer runs factory air:fuel mix based on engine load(MAP sensor) and throttle position(TPS), in Open Loop
It also has an "offset" for Open Loop based on Closed Loop operation for the last 100 or so cycles, in newer computers it is Long Term Fuel Trim, so if fuel pressure is lower and there are a few small vacuum leaks this will "offset" the factory table mixes so engine runs the same, when brand new or 25 years old, in theory
If you haven't already I would replace ECT sensor
ECT(engine coolant temp) sensor is only used by the computer
Dash board gauge uses a SENDER, 1 wire
If ECT sensor is telling computer coolant is -20degF at cold start, and actual temp is 50degF, then engine won't run well until ECT reports correct temp.
ECT sensor tells computer what Cold Idle RPM to set and what Richer fuel mix to use, all cold engines need to be "Choked", run a richer mix and higher idle, fuel injection doesn't have Jets so can't use a Choke plate, so computer has to emulate "Choke"
And how much Choke to use is based on engine/coolant temp, same as it was on a carb
If you have replaced ECT then check voltages on its two wires
Use a sewing needle/pin to pierce one wire at a time
Key on
One wire will have 5volts, that the Reference voltage from the computer
Other wire will have Temp Voltage, called Return voltage
Look here: http://www.explorerforum.com/photopost/data/503/41928895.gif
50degF = 3.5v
140degF = 1.3v
Voltage will go down as coolant warms up
So check it cold then run engine for 2 or 3 minutes and shut it off then check it again
Then at full warm up check it
If you can check it cold with engine running, be careful, then you may see Return voltage jumping around, computer will react instantly to that so would account for wandering idle and Rich mix when cold
Could be 5v wire is shorting to Return wire, causing high and low voltage on return wire
But only O2 sensors are not used, O2 sensors need to be at or above 600degF to read oxygen levels correctly, which is why O2s are also heated, to get them up to temp ASAP
Computer runs factory air:fuel mix based on engine load(MAP sensor) and throttle position(TPS), in Open Loop
It also has an "offset" for Open Loop based on Closed Loop operation for the last 100 or so cycles, in newer computers it is Long Term Fuel Trim, so if fuel pressure is lower and there are a few small vacuum leaks this will "offset" the factory table mixes so engine runs the same, when brand new or 25 years old, in theory
If you haven't already I would replace ECT sensor
ECT(engine coolant temp) sensor is only used by the computer
Dash board gauge uses a SENDER, 1 wire
If ECT sensor is telling computer coolant is -20degF at cold start, and actual temp is 50degF, then engine won't run well until ECT reports correct temp.
ECT sensor tells computer what Cold Idle RPM to set and what Richer fuel mix to use, all cold engines need to be "Choked", run a richer mix and higher idle, fuel injection doesn't have Jets so can't use a Choke plate, so computer has to emulate "Choke"
And how much Choke to use is based on engine/coolant temp, same as it was on a carb
If you have replaced ECT then check voltages on its two wires
Use a sewing needle/pin to pierce one wire at a time
Key on
One wire will have 5volts, that the Reference voltage from the computer
Other wire will have Temp Voltage, called Return voltage
Look here: http://www.explorerforum.com/photopost/data/503/41928895.gif
50degF = 3.5v
140degF = 1.3v
Voltage will go down as coolant warms up
So check it cold then run engine for 2 or 3 minutes and shut it off then check it again
Then at full warm up check it
If you can check it cold with engine running, be careful, then you may see Return voltage jumping around, computer will react instantly to that so would account for wandering idle and Rich mix when cold
Could be 5v wire is shorting to Return wire, causing high and low voltage on return wire
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